I really think a seat from a conversion van or RV (GM chassis - I believe these use standard GM seat frames) would be best for this particular situation. Air ride would be nice, but not feasible with this vehicle. They already have hydroboost brakes because there's no room for a vacuum booster. Try fitting an air compressor into the accessory drive. Not happening. Maybe an electric air-ride control compressor from a late-model Buick, Olds or Caddy would be up to the task, still a lot of work for not much more benefit than you would get from conversion van seats. Also consider that anything bigger than what this one came with will likely be a nightmare when servicing the engine -- most repair and some maintenance on the engine in the van chassis used here is done through a doghouse cover at the rear of the engine, built into the dash. You don't want to have to remove the seat just to change a distributor cap or ignition coil.
I can tell you that the leather seats in the '98 GMC Safari I used to own were quite comfortable, and likely interchange with a Savana, even if you have to swap the bottom seat mounts. Mine was top-of-the-line and had captain's chairs in the front / middle row, there are plenty of these in the boneyards, Astro / Safari / Savana / Express alike. Both Savanas and Safaris were available with leather and six-way power lumbar. My van was used as a taxi, so that speaks volumes for the comfort. Express / Savana seats will also bolt up like the original with little to no effort -- a major consideration in this case, IMO. I believe Astro / Safari seats will fit as well, but you may likely have to swap the seat chassis where it mounts to the floor (usually 4-6 Torx bolts).
I would look for Express and Savana conversion vans, and look at a few Astros and Safaris as a backup plan, they will likely fit fine, even if the bottom mount frame to the floor has to be swapped with your original. It's a lot easier than it sounds, believe me -- I've done this on a Venture van before, though I was replacing an incorrect seat for the position that had been forced, requiring me to get the seat mount from one yard, and the actual seat from another (incorrect fabric, good mount -- correct fabric, damaged mount), using the same models. Only took me a couple hours, including fighting with the stubborn Torx bolts that were EXTREMELY overtorqued. Unless an upgrade isn't imperative, skip the windowless cargo / work vans, they will have standard vinyl or cloth like you already have.
UPDATE: I did some poking around and the Express / Savana seat does look larger than a Safari seat, so I would recommend sticking to Savana / Express conversion van donors.
